Django 2.0 ImportError fix (pytest-dev#518)

Author: jpic

pytest_django/plugin.py

@@ -446,7 +446,11 @@ def _django_set_urlconf(request):
     if marker:
         skip_if_no_django()
         import django.conf
-        from django.core.urlresolvers import clear_url_caches, set_urlconf
+        try:
+            from django.urls import clear_url_caches, set_urlconf
+        except ImportError:
+            # Removed in Django 2.0
+            from django.core.urlresolvers import clear_url_caches, set_urlconf
 
         validate_urls(marker)
         original_urlconf = django.conf.settings.ROOT_URLCONF